1. Field of the Invention
The present invention relates to developing compositions used in developing an electrostatic latent image by an electrophotography process, electrographic recording method, or electrographic printing method or the like.
As electrophotography developing methods, there exist a cascade developing method, a magnetic brush developing method, and the like. Characteristics required for the toner and the carrier used in these methods include, characteristics such as possession of a toner of appropriate degree of triboelectric charging, uniform particle size, stabilization of the surface condition using temperature and the like by various unchanging characteristics, and the like.
A various materials have been used in prior art as the carriers and the toner which satisfy the above characteristics. However, recently ferrite carriers having a stable charge characteristic are being widely employed.
Ferrite carriers used in the past, in general, possessed the following characteristics: a particle size distribution was 40 to 180 .mu.m; a large average particle diameter of approximately 100 .mu.m; basic composition containing BaO, ZnO, CuO, MgO, MnO, and the like, with Fe.sub.2 O.sub.3 as the main component; an electrical resistance rate of 10.sup.3 to 10.sup.12 ohm cm; a saturated magnetization value of 10 to 80 emu/g in an applied magnetic field of 3000 oersted; and particle surfaces coated with an oxide or a resin film.
However, recent copy picture images increasingly require high definition and high picture image quality. In order to satisfy these needs, there is growing demand for ferrite carriers having small particle size with the range of distribution of the particle diameter ranging from 30 to 150 .mu.m and with an average particle diameter of 50 to 60 .mu.m.
However, because almost all of the ferrite carriers having small particle size, such as these having an average particle diameter of less than 60 .mu.m, were barely magnetized, the ferrite carriers separated easily from the developing sleeve, and adhered to the surface of the photoconductor, or jumped out of the developing machine. Therefore problems occurred such as damage to the photoconductor and the photoconductor cleaning blade, dirtiness falling on the picture image and into the copy machine, and the like.
Consequently, in order to solve these problems, particles having a particle diameter of less than 60 .mu.m had to be removed. Because of this it is very difficult to obtain by a conventional techniques a high quality picture image using ferrite carriers consisting of small size particles.
Therefore, in order to obtain a high quality picture image having high definition by using prior art ferrite carriers with an average particle diameter of approximately 100 .mu.m, the volume average particle diameter of the toner must be decreased to 10 .mu.m or less. However, lowering the particle diameter of the toner gives rise to problems such as increased dependency on the environment, instability due to rising of electrical values of the toner, a decrease in the fluidity of the toner, degradation of anti-heat longevity, a decrease in the density of the picture images, an increase in background contamination, and the existence of poor cleaning properties.